The goal of this three year study is to estimate the relative contributions of host genetic and environmental factors to gingivitis, probing depths, periodontal attachment levels and the microbial composition of supra and subgingival plaque in humans. Chronic periodontitis is a major cause of tooth loss in adults. Millions of dollars have been spent for research directed at its apparent environmental etiology and billions of dollars have been spent for treatment. This study will utilize the uniquely simple, but powerful design of twin studies to address the critical lack of knowledge regarding the contribution of genetic factors to the etiology and pathogenesis of destructive periodontitis. Four specific types of middle-aged adult twins will be identified for participation in this study through The Minnesota Center for Twin and Adoption Research. These include: 1) 105 pairs of identical twins reared together, 2) 105 paris of fraternal twins reared together, 3) 16 pairs of identical twins reared apart and, 4) 8 pairs of fraternal twins reared part. Selection for participation will be based upon age, gender, availability, and willingness to volunteer. A medical and dental history will be obtained and, using selected index teeth, each subject will be independently evaluated for the prevalence and severity of periodontal disease by 2 trained and calibrated clinical investigators. Examiners will be blind to subject classification and each other's measurements. Supra- and subgingival plaque samples from each index site will be documented for the distribution of bacterial morphotypes by darkfield microscopy. Aliquots of plaque, saliva, and serum will be frozen and available for future analysis if a significant genetic contribution to the etiology or pathogenesis of periodontal disease is found. Data from clinical measurements and darkfield microscopy will be analyzed using analysis of variance for patients mean values and correlation coefficients for site specific data to estimate the relative contributions of genetic and environmental factors to the determination of periodontal disease and bacterial colonization of plaque. In addition, by using within and between pair mean squares in model fitting techniques, we shall be able to estimate for each dependent variable, the contribution of genetic, shared environmental, and unshared environmental influences.